The present invention is generally related to measurement of eye movements, and in particular embodiments provides methods, systems, and devices for measuring the position of the eye relative to diagnostic devices and/or treatment devices such as laser systems for refractive surgery of the cornea or other parts of the eye.
Ophthalmic diagnostic devices, such as Topography, Pachymetry, Optical Coherence Topography (OCT) and Wavefront sensing systems measure the shape, thickness and optical parameters of different surfaces of the eye. With the advances in methods, systems and devices in the ophthalmic diagnosis an accurate measurement of the exact location of each diagnostic measurement on the eye is highly desired in order to combine, compare or map succeeding measurements with the same or different devices over time together. Some techniques, such as OCT provide only one measurement (i.e. distance and thickness of the cornea) at a specific location on the eye at a time. In order to allow an assessment over a specific section (line) or are area, several measurements are taken consecutively at different locations on the eye over a certain period of time by scanning the diagnostic measurement device over the eye.
Ophthalmic treatment devices, here in more specific laser systems, perform treatments on different surfaces of the eye (i.e. cornea, lens, iris or retina). In refractive surgery, laser systems are used to achieve a desired change in corneal shape, with the laser removing thin layers of corneal tissue at specific locations on the cornea using a technique generally described as ablative photodecomposition. Laser eye surgeries are useful in procedures such as photorefrative keratectomy, phototherapeutic keratectomy, laser in situ keratomileusis (LASIK), and the like. Newer Femto-second laser systems perform specific procedures on the cornea to create flap on the cornea or perform direct treatment with the corneal material. Laser treatment procedures require several specific ablations at a defined position on the eye over the treatment time to create the intended result. The laser may or may not be directed towards different locations onto the eye and the laser beam may be modified in size, form (i.e. slit, circular) and energy profile throughout the ablation procedure.
To position a diagnostic measurement or treatment procedure onto the eye, the location of the eye needs to be known. The eye position may therefore be adjusted according to the instrumentation to align the eye in a defined position relative to the optical axis of the diagnostic or treatment system. During the procedure, which may take seconds or minutes, the patients eye or head can move the away from this initial aligned position. Therefore, the ability to automatically track or follow the eye position throughout the diagnostic or treatment procedure is recognized as a highly desirable, if not a necessary feature within these systems.
Movements of the eye include voluntary and involuntary—primarily rotational—movement of the eye in the head. Even if the patient is cooperative and can sharply visualize and fixate on a specific fixation target, certain eye movement will still occur, such as eye rotation in yaw (horizontal), pitch (vertical) and roll (torsion). Head motion can also occur during the treatment, resulting primarily into a horizontal and vertical translational movement relative to—and rotational movement around—the optical axis of the diagnostic or treatment system. In specific treatment procedures, such as treatment of irregular astigmatism or cutting the flap with a femtosecond laser system the absolute translational and rotational position of the eye in all six dimensions is required relative to the treatment system for accurate and secure treatment.
Therefore, tracking the eye has been proposed to avoid uncomfortable structures, which attempts to achieve total immobilization of the eye and locate the eye at a defined position relative to the diagnostic or treatment device. A variety of structures and techniques have been proposed for tracking the eye during the diagnosis and/or treatment, and to position the diagnostic measurement or treatment position to a certain position on the eye. For this purpose a sensor device fixed relative to the diagnostic or treatment system observes the eye or its specific features. Two different general approaches, Closed-loop and Open-loop tracking Eye Tracking Methods have been introduced.
Closed-Loop Eye Tracking Methods provide a horizontal and vertical stabilization of an optical projection of the eye towards the diagnostic or treatment system. Movements of the eye are sensed by means of detecting one or multiple specific feature of the eye (i.e. mostly a specific section of the pupil iris boundary is used) with a sensor device via a position controllable x-y mirror device. The sensor provides an position error signal if the tracked feature of the eye is moved, which is then used by a controller to create a feedback positioning signal to control an x-y mirror position to project the tracked feature back onto the same location on the sensor device. This technique is also called closed loop tracking and performs a stabilization of the target relative to the sensor. If the sensor device is mounted fixed to the diagnostic and treatment device, the projected image of the eye is stabilized relative to the diagnostic or treatment device. The sensor with its applied method senses a deviation of the projected eye from its indented stabilized position and controls the mirror to project the eye back into the intended stabilized position. A measurement of the actual x and y position may be obtained indirectly from the control output positioning the x-y mirror device.
One specific implementation of these Closed-loop Eye Tracking Methods is described in the patent U.S. Pat. No. 5,632,742 (Eye Movement Sensing Method and System, Frey et al.), hereafter called LADAR tracker, which applies through an motorized x/y mirror device sequentially 4 light spots onto 4 different locations of the pupil-iris boundary, and measures the returned light from each location. Eye motion relative to the light spots result into a change of brightness returned by each spot caused by different light energy reflected by iris and pupil. This analysis of the returned light intensity by each of the four spots provides an error position signal used to control the motorized x-y mirror position to reposition the spots centered on the pupil-iris boundary. As a result the x-y mirrors are always in a fixed orientation relative to the pupil-iris boundary, which the laser treatment device can now use to project its ablation laser spot stabilized onto the eye. A relative positioning of the treatment location onto different locations onto the eye can be accomplished using a second set of controllable mirrors. Limiting the analysis of the eye to the intensity of light returned from 4 small discrete areas of the eye, allows fast processing and positioning of the mirrors, to stabilize the projection of the eye for treatment even during fast eye movements.
To initiate the tracking with this technique the pupil size needs to be known to adjust the relative position of the spots onto the pupil-iris boundary, which requires manual or semi-automatic adjustment procedures. Furthermore, the pupil size needs to be constant throughout the procedure, since the light spots projected onto the pupil-iris boundary are fixed relative to each other. However, pupil size changes generally occur and therefore need to be omitted as much as possible by dilating the pupil pharmaceutically before the treatment. This requires another treatment step in the overall procedure and creates uncomfortable temporary side effects for the patient (less visual acuity during the dilation period) and can influence the clinical outcome of the diagnostic or treatment procedure. Firstly, widening the pupil—the target to be tracked—is not symmetrical relative to any fixed point on the cornea—the target to be treated—and therefore a positioning error may occur. This may be compensated with a specific calibration procedure. Secondly, dilation may change physical characteristics of the eye, which then may affect the treatment process (i.e. cutting the flap) itself.
Another technique of Closed-loop Eye Tracking Methods combines the optical technique of Confocal Reflectometry with the electronic technique of phase-sensitive detection, hereafter called CRP Tracker, as described in the patent U.S. Pat. No. 5,943,115. It utilizes a high-bandwidth feedback signal derived from the light of a low-power “tracking beam” scattered off the surface of the tracked object (i.e. retina or iris of the eye). The tracking beam is directed onto the tracked surface of the eye by fast x-y position controlled tracking mirrors. The feedback signal continually adjusts the mirror orientations to lock the tracking beam to a target on the tracked surface of the object and the tracking mirror surfaces follow the motion of the tracked surface of the object. The diagnostic or treatment device may therefore be applied fixed to the eye through the tracking mirrors. Relative positioning of a diagnostic or treatment location onto different location onto the eye can then be accomplished using a second set of controllable mirrors.
One benefit of the CRP tracker is, that it tracks only a single small target area, which provides sufficient contrast changes, i.e. a specific area of the iris or retina. This eliminates the need of relative positioning of several areas and compensation of distances during the procedure as need with the LADAR tracker. Although the CRP tracker has been primarily applied for tracking of retinal features for diagnosis and/or treatments of the retinal surface, this technique may be applied to track a feature close to the surface to be diagnosed or treated. As with the LADAR tracker, this technique provides no automated method to identify which feature on which surface shall be tracked. In addition, there is no objective control available that a specific feature is lost or another similar feature clos by is tracked, which would result into a position error.
The above-described Closed-loop Eye Tracking Methods provide a fast two dimensional tracking and stabilization of the projected eye to the diagnostic or treatment device. However, the tracking of the eye is performed on only specific features undergoing both translational and rotational movement of the eye. These methods cannot discriminate between translational and rotational movement of the eye, which is becoming recently of more interest. Furthermore, the distance of the eye relative to the diagnostic and/or treatment device is not measured and torsional rotations of the eye are either not detected (LADAR tracker) or may create an error in horizontal and vertical tracking (CRP tracker). Furthermore, the introduction of other objects into the field of view such as surgical instruments occluding the tracked featured may create a false measurement or loss of tracking.
Open-Loop Eye Tracking Methods sense the eye directly or via a fixed mirror system, and process the sensor information to identify a specific feature and its location in the sensor information.
The most common approach of Open-Loop Eye Tracking Methods, hereafter called VIDEO tracker, uses imaging devices, i.e. a CCD camera, which is mounted in such a way that it observes the eye within the optical axis of the diagnostic or treatment device. The eye is illuminated with infrared light from light sources which are mounted non-coaxial from the optical-axis. Using infrared filters the imaging device integrates an image of the eye from the infrared light, which provides a higher image contrast between the dark pupil and surrounding iris and sclera than with other visible light. The obtained images are transferred to an image processing system where each image is digitized in picture elements (pixels) and processed to determine the center of the pupil. In these systems the pupil is detected as a circular formed dark area within an otherwise brighter image of the eye from the iris and sclera. Detection of the pupil area is performed using a brightness threshold to detect all pixels, which are below this threshold. Thereafter, all pixels may be analyzed for horizontal and vertical connectivity to other pixels, which are below this threshold, resulting in an identification of several objects containing connected pixel elements, which are below this threshold level. All objects are thereafter analyzed according to several geometric parameters to identify the pupil. If an object in the image fulfils all these geometric requirements for a pupil, the center of gravity (COG) or other geometric calculations are preformed to obtain a center position from this pupil object.
The obtained horizontal and vertical pupil position relative to the optical axis is provided to the diagnostic and/or treatment system as horizontal and vertical position of the eye during the procedure. This information is then used in different ways, depending on the requirements from the diagnostic and/or treatment procedure, ranging from only registering where a diagnostic measurement or treatment was performed on the eye, performing a diagnostic measurement or treatment only within a certain position range of the eye, or offsetting the diagnostic measurement and/or treatment position with the eye position using a x/y mirror system. The latter case is often used for example in scanning laser system, where the eye position is used to offset to the indented scanning position of the treatment.
An improvement towards the above-described VIDEO tracker has been proposed in Patent U.S. Pat. No. 6,322,216, where 2 off axis imaging devices are used to overcome challenges integrating the imaging devices within the optical path. The images of each imaging device may be used to determine the overall horizontal and vertical position of the eye from the perspective of each camera. However, due to the off-axis viewing of the eye, a change of eye distance relative to the laser system—even along the optical z axis with no change of horizontal and vertical position—results in a different horizontal and vertical position measurement obtained by each off axis imaging device. To overcome this limitation the position obtained from both imaging devices must be combined in order to determine also the distance of the eye relative to the laser device, allowing a means of correcting the parallax error and providing a correct horizontal and vertical position of the relative to the treatment device. Therefore, if depth changes of the eye relative to the treatment device can occur, always the image analysis of both imaging devices is needed for an accurate measurement of horizontal and vertical position.
VIDEO trackers have been proven effective for several applications in diagnostic and/or treatment applications. In the field of refractive surgery, VIDEO trackers currently have several advantages and limitations compared with the other Eye Tracking Methods. An advantage of the VIDEO tracker over the LADAR tracker is, that VIDEO tracker can track the pupil at different sizes of the pupil. This advantage however has a certain limitation, since pupil size changes do not occur symmetrically relative to the cornea, which may creates a positioning error on the cornea at different pupil sizes. The setup of video trackers is simpler and can be automatic, however the speed of the VIDEO tracker is limited to the image rate of the sensors and the processing of the image. More specificly, the time needed to integrate an image on the sensor, to transfer the sensor information to the processing unit, and to process the image to obtain the pupil position information, results in a latency of position information within which the eye may continue to move, resulting in a dynamic positioning error of a succeeding diagnosis or treatment. This latency has been minimized using faster image sensors with faster image rates to have approximately the same overall latency period as with the LADAR tracker.
Although the known Eye Tracking devices have proven effective and safe for the current state of art in diagnostic or treatment of the eye, recent improvements and developments in ophthalmic diagnostic and/or treatment devices as well as the procedures involved using this technology have an increased demand on resolution, accuracy, dimensions, robustness and security for the registration of eye position and to control for it change during the procedure. This demand cannot be fulfilled by the current Eye Tracking Methods, primarily limited by its overall simplified measurement of a “projected” pupil based position onto a sensor device and not taking into account the different translational or rotational state of the eye in space relative to the diagnostic and/or treatment device.